1. Field of the Invention
This invention relates to asymmetric cellulose acetate membranes and their use for desalination, nonaqueous liquid separation, ultrafiltration, pervaporation and gas separation. More particularly, the invention relates to a method for converting asymmetric membranes from their aqeuous state to the dry state by direct evaporation of water. In another aspect, the invention relates to the use of these membranes to separate liquid or gaseous mixtures into various fractions and to their use for reverse osmosis, ultrafiltation, and pervaporation.
2. Description of the Prior Art
Loeb and his co-workers disclose in, for example, U.S. Pat. No. 3,133,132 a method for preparing a modified cellulose acetate membrane for desalination of water by first casting a solution of cellulose acetate as a thin layer, and then forming a dense membrane skin on the thin layer through various techniques such solvent evaporation followed by quenching in cold water. Many research programs on reverse osmosis membranes have been very actively conducted. Reverse osmosis is now widely avaialble for many industrial applications such as desalination, pollution control, water reclamation, food processing and many other separation concentration and recovery processes.
Separation of gases by permeation through polymer membranes has generated considerable interest in recent years. Although the process has been known for over a century, recent advances are making it economically competitive in many areas. One of the earliest permeation studies was done by G. J. Van Amerongen, "The Permeability of Rubberlike Substances to Gases," Communication No. 46, Rubber Stichting Rev. gen caoutchouc 21, 50-6 (1944). He measured the permeability of nine rubbers to hydrogen, helium, oxygen, nitrogen, carbon dioxide and methane and independently determined solubilities and diffusivities which related to the nature of gases and rubbers. William J. Ward III and Charles K. Neulander, U.S. Clearinghouse Fed. Sci. Tech. Inform. PB Rep. 1970, No. 191769, 15 pp (Eng.) chose immobilized liquid membranes such as polyethylene glycol as a viable alternative. Some liquids have much larger diffusion coefficiants than solid polymers and may also have enormous solubilities for reactive gases. However, although the resulting polyethylene glycol membrane had shown excellent selectivity, the permeation rate was an order of magnitude too low for practical applications.
In addition to homogenous polymer membranes and immobilized liquid membranes, a third type of membrane that has been considered for gas separations is again the asymmetric membrane which was based on knowledge developed concerning liquid-liquid separation membranes by Loeb and Sourirajan. Jitendra P. Agrawal and Sourirajan Srinivasa, J. Appl. Polym. Sci. 1970, 14 (5), 1303-21 (Eng.) studied the permeation of pure gases and several binary gas mixtures through porous freeze-dried cellulose acetate membranes. Due to the porosity of the membrane, separation factors were rather low but the permeation rates were reasonable. Schell, William J.; Lawrence, Ralph W. and King, William M., United States Energy Research and Development Administration Contract No. E (149-18-2000)). William J. Schell, Ralph W. Lawrence and William M. King, United States Energy Research and Development Administration Contract No. E (149-18-2000) reported asymmetric cellulose acetate membranes which had selectivities characteristic of nonporous cellulose acetate films but much higher permeation rates. They used these membranes to separate hydrogen from methane and carbon monoxide and suggested applications in coal utilization processes.
As is well known to those skilled in the art, the membranes prepared by the techniques of Loeb and Sourirajan must be kept wet or in the case of gas separation, dried through special drying processes. If they are allowed to dry under ambient conditions, they undergo compaction and suffer a non-recoverable loss in permeation rate and selectivity. It is believed that the collapse of the membrane porosity is due to surface-tension related forces between the water and the membrane. These are so great that as the water globules within the membrane decrease in size, the pores around them collapse. Therefore, special drying processes are necessary for application to gas separation in order to prevent the membrane from shrinking and losing its asymmetric character as it does when the water is simply allowed to evaporate.
Many workers have attempted to dry asymmetric cellulose acetate membranes directly from the water-wet state that results from their being cast and gelled in water. Prior to this invention, all attempts to dry the membrane directly from water resulted in shrinkage and total flux loss, or involved the use of surfactants and generally humectants as well that were incorporated into the membrane by soaking it in an aqeous solution of the chemical prior to drying. This method worked fairly well when the membrane was used in reverse osmosis since the chemicals were washed out with the water when the membrane was put into service. It is not suitable for drying gas separation membranes, however, since the residue plugs the pores and renders the membranes impermeable to gases.
Only a few efforts to freeze-dry asymmetric membranes for gas separation have been made. This method is expensive and the membranes generally have impaired performance.
The drying method that is most widely used today is solvent exchange. In this approach, the water-wet membrane is soaked successively in a series of solvents so as to end up with a solvent that has little interaction with the polymer and thus will not affect the membrane in an adverse manner as it evaporates.
The general scheme has been to replace the water by soaking the membrane in an alcohol such as isopropanol alcohol (IPA). Then the alcohol, which would still cause the membrane to collapse if the membrane were dried from it, is often replaced with hydrocarbon such as hexane. At this point, the membrane can be dried without damage or loss of performance. The hexane is sometimes exchanged with a non-flammable liquid such as Freon prior to the final drying.
Accordingly, this invention pertains to a direct air-dried asymmetric cellulose acetate membrane prepared by an improvement in the Loeb et al technique. These seslectively permeable membranes are directly dried from their annealing step by evaporation of water. In general, these air-dried membranes exhibit the same gas permeation rates and high separation factors as cellulose acetate membranes dried by solvent exchange. Moreover, the membranes of this invention can be rewetted to provide desirable flux rates and selectivities in brackish water desalination by reverse osmosis. The advantages of the dry reverse osmosis membrane are several, including easier handling and increased storage life because of no bacteria growth. Furthermore, this invention provides a method for making direct air-dried cellulose acetate membranes in one preferred embodiment wherein the asymmetric structures are preserved by adding the hydropobic chemical drying agent to the casting solution.